1. Field of the Invention
The present invention relates to an apparatus for the heat treatment of synthetic yarn comprising at least one curved heating surface, where the length over which the yarn engages said heating surface can be changed.
2. Brief Description of the Background of the Invention Including Prior Art
British patent specification No. 883 613 shows an apparatus of this kind where the yarn is guided twice along the heating surface such that it engages this surface over the total length of the surface during the first run, but engages only part of the surface during the second treatment. The yarn is fed with constant speed to the heater, and the length at which the yarn engages the heater surface during the second treatment can be adjusted by means of an adjustable feed roller.
A heater may be used where the yarn is guided along a slightly curved heating surface in order to supply the heat required for stretching or setting a running yarn. This heating surface may be heated electrically or by means of steam. In known heating apparatuses, the heating surface forms part of the wall of a heating vessel which is partially filled with a heat transfer liquid and partially filled with vapor of said liquid. This type of heater warrants a uniform temperature along the entire length of the heating surface. The amount of heat transferred to the yarn mainly depends on the temperature of the heating surface and on the period of time during which the yarn engages the heating surface. This period of time is determined by the length of the heating surface and by the speed of the yarn.
During the treatment of the yarn, the speed of the yarn and the temperature of the heater normally are kept constant. During the starting period of the treatment process and in the case of a yarn break, the speed of the yarn changes essentially, and therewith the period of time changes during which the yarn engages the heating surface. In order to prevent overheating of the yarn during the starting period, the temperature of the heater was raised in accordance with the increasing yarn speed, and in the case of a yarn break the supply of energy to the heater was interrupted immediately. The thermal intertia of the heater, however, does not permit a continuous alignment of the heat supply to the instant speed of the yarn without delay.